Catalytic cracking processes utilize zeolite-containing catalysts to produce light distillate fractions, such as gasoline, from heavier hydrocarbon feedstocks. Deterioration of the cracking catalyst occurs when metal contaminants introduced into the fluid catalytic cracking (FCC) zone with the feedstock deposit on the catalyst. These metal contaminants result in zeolite destruction and catalyst deactivation, particularly under the oxidizing, high temperature (&gt;1300.degree. F.) conditions used to regenerate the catalyst after the cracking zone. Contaminants in FCC hydrocarbon feedstocks are both naturally occurring in the crude oil and picked up during processing. Heavy metals, such as nickel, vanadium, iron and copper, usually occur naturally in the crude oil and are concentrated in the residual bottom streams from atmospheric and vacuum distillation processes. Iron and copper contaminants are also picked up in the residual oils during processing due to corrosion and erosion of vessels and transfer lines. Alkali metals (i.e., lithium, sodium, potassium, rubidium or cesium) are naturally occurring in crude oil and their concentration is usually reduced significantly by desalting processes. However, rapid fluctuations in feed quality can lead to desalter upsets with significant amounts of alkali metals carrying into down stream processing. Also, off specification streams or residual oils are frequently added to FCC feedstocks to reduce waste resulting in increased levels of alkali metals. Caustics, such as sodium hydroxide, are frequently used at several points in the refinery for chloride neutralization. The NaCl crystals thus formed concentrate in the bottoms which are frequently added to FCC feedstocks. Additional use of alkali metal contaminated water or steam around an FCC unit can contribute added contaminants to the catalyst.
Alkali metals deactivate FCC catalysts by two known mechanisms. The first results in the loss of cracking activity due to neutralization or poisoning of the acid sites. The alkali metal oxides can also combine with silica and/or alumina present as part of the catalyst matrix to form salts. Usually enhanced by the presence of steam, these high temperature fluxing reactions result in zeolite destruction and reduced catalytic activity causing a decrease in the relative amount of feedstock that is converted to lighter distillate fractions, such as gasoline.